This invention relates to a method of consecutively bending at least two glass sheets to be united into a curved and laminated glass sheet such as an automobile windshield and annealing the bent glass sheets and an apparatus for the bending and annealing method.
Many of recent automobiles have a curved windshield made of laminated glass, and further there is a trend toward the use of a curved and laminated glass pane in the rear window.
Glass sheets for a curved windshield of laminated glass are individually bent to a curved shape as specified at a temperature near the glass softening point and then annealed. The fundamental object of the annealing is for relieving the bent glass sheets from internal strains, and it is also intended not to strengthen the glass sheets so that a human head colliding against the windshield may not receive a very dangerous shock. Besides, the annealing prevents the glass of the windshield from breaking into very small pieces and obstructing the driver's field of view in case of an accident.
In annealing two (or more than two) curved glass sheets to be united into a laminated glass member such as an automobile windshield, it is desirable to place one glass sheet on the other in order that the curvature of the former glass sheet may accurately conform to that of the latter glass sheet. In this connection, JP-A No. 63-190730 shows a process comprising the steps of bending two glass sheets individually and consecutively, placing each of the resultant curved glass sheets on a ring-like holder and annealing the glass sheets on the respective holders, thereafter taking one of the curved glass sheets off the ring-like holder and placing that glass sheet upon the curved glass sheet on the other ring-like holder and subjecting the two curved glass sheets in the stacked state to a shape redressing heat treatment. For this process the annealing furnace needs to have a large capacity to accomode at least two curved glass sheets each placed on the ring-like holder, and a complicated system is needed for the transfer of the ring-like holders from the glass sheet bending station to the annealing furnace, then to the shape redressing heat treatment station and again to the bending station.
JP-A No. 62-283834 shows a process comprising the steps of bending a first glass sheet and placing the resultant curved glass sheet on a ring-like holder mounted on a carriage, bending a second glass sheet while maintaining the first curved glass sheet nearly at the bending temperature and placing the second curved glass sheet upon the first curved glass sheet on the ring-like holder, transferring the carriage carrying thereon the ring-like holder and the curved glass sheets to an annealing furnace, transferring the annealed glass sheets to a next station and returning the carriage to the bending station. In this method a matter for consideration is the apparatus for the cycling transfer of the carriage, and problems to be solved include a considerable amount of thermal energy consumed in reheating the carriage and deformation of the repeatedly heated and cooled carriage.